The invention relates to rheology enhancers for viscoelastic surfactant fluid systems (VES's). More particularly it relates to selection and optimization of rheology enhancers for fluid systems to be used over broad ranges of salinity and temperature. Most particularly it relates to rheology enhancers to increase stability and shorten shear recovery times of VES's for use in oilfield treatment fluids.
Certain surfactants, when in aqueous solution, form viscoelastic fluids. Such surfactants are termed “viscoelastic surfactants”, or “VES's”. Not to be limited by theory, but many viscoelastic surfactant systems form long rod-like or worm-like micelles in aqueous solution. Entanglement of these micelle structures gives viscosity and elasticity to the fluid. For a fluid to have good viscosity and elasticity under given conditions, proper micelles must be formed and proper entanglement is needed. This requires the surfactant's structure to satisfy certain geometric requirements and the micelles to have sufficient length or interconnections for adequate entanglements. VES fluid systems may contain many additives, such as but not limited to co-surfactants and salts, that perform such functions as increasing the stability (especially thermal stability) or increasing the viscosity of the systems by modifying and/or stabilizing the micelles.
Many chemical additives are known to improve the rheological behavior (greater viscosity and/or greater stability and/or greater brine tolerance and/or lower shear sensitivity and/or faster rehealing if micelles are disrupted, for example by shear). Such materials are typically called co-surfactants, rheology modifiers, or rheology enhancers, etc., and typically are alcohols, organic acids such as carboxylic acids and sufonic acids, sulfonates, and others. We shall use the term rheology enhancers here. Such materials often have different effects, depending upon their exact composition and concentration, relative to the exact surfactant composition (for example hydrocarbon chain lengths of groups in the surfactant and co-surfactant) and concentration. For example, such materials may be beneficial at some concentrations and harmful (lower viscosity, reduced stability, greater shear sensitivity, longer rehealing times) at others.
In particular, many VES fluid systems exhibit long viscosity recovery times after experiencing prolonged high shear. Slow recovery negatively impacts drag reduction and proppant transport capability, which consequently lead to undesirably high treating pressures and risks of near wellbore screen-outs. Although additives are known that can shorten VES shear recovery times and increase viscosity stabilities, there is a need for additional simple, inexpensive rheology enhancers.